Fit function toolbar improvements

[Nick Draper]

Here are a few bugs/requests for the fit function dialog

1) The plot guess button only shows up if the select multiple peaks button is clicked.  It should show up as soon as a function is defined
2) Pressing fit with an Intensity of 0 causes a seg fault.  It should raise an error flag instead.  I only tested this with the Ikeda-CarpenterPV function
3) Pressing fit when all the parameters are fixed gives an error.  My instinct says it should work returning a chisquare.  Maybe the cleaner operation would be
to add a single iteration button and an error message if all the parameters are fixed.  The purpose of  single iteration button is so you can calculate the chisquare and the covariance matrix 
for your guess function.
4) More advanced fitting interface.  I think there should be an advanced user function that will execute a piece of python code.  It means the users will have to conform to an api, but it should give a link to all the glory of python,
ie, gamma functions, bessel functions,  wrapping C code, etc.

Garrett

[Nick Draper]

Number 4 should not be attempted under this ticket.

[Nick Draper]

Bulk move of tickets at the end of iteration 28

[Nick Draper]

Accepted and assigned tickets moved at iteration 29 code freeze

[Roman Tolchenov]

In [13882]:

Fit doesn't crash if called with a fixed function but returns a workspace with calculated values and the chi squared. re #2537

[Nick Draper]

Bulk move of tickets to iteration 31 at the iteration 30 code freeze

[Roman Tolchenov]

I should have read the first comment earlier...

[Andrei Savici]

Point 1 is not fixed

[Vickie Lynch]

Points 1 and 4 should be tickets for next iteration. Points 2 and 3 are fixed.

[Stuart Campbell]

This ticket has been transferred to github issue ​3384